The field of invention relates to wireless communication generally; and more specifically, to canceling an offset in a received signal.
FIG. 1 shows a portion 106 of a receiving device 166 referred to as a demodulator. A demodulator 106 provides a signal (commonly referred to as a baseband signal b(t) in various applications) that is representative of the information being sent from a transmitting device 165 to a receiving device 166. The demodulator 106 extracts (i.e., demodulates) the baseband signal b(t) from a high frequency wireless signal that xe2x80x9ccarriesxe2x80x9d the baseband signal b(t) through the medium (e.g., airspace) separating the transmitting and receiving devices 165, 166.
The particular demodulator 106 example of FIG. 1 is designed according to: 1) a demodulation approach that is commonly referred to as super heterodyne detection (hereinafter referred to as a heterodyne detection for simplicity); and 3) a modulation/demodulation scheme referred to as Frequency Shift Keying (FSK). The industry standard referred to as xe2x80x9cBLUETOOTHxe2x80x9d (the requirements of which may be found in xe2x80x9cSpecification of the Bluetooth Systemxe2x80x9d, Core v.1.0B, Dec. 1, 1999, and published by the Bluetooth Special Interest Group (SIG)) can apply to both of these approaches and, accordingly, will be used below as a basis for reviewing the following background material.
Heterodyne detection is normally used when dedicated channels are allocated within a range of frequencies 111 (where a range of frequencies may also be referred to as a xe2x80x9cbandxe2x80x9d 111). For BLUETOOTH applications within the United States, 89 channels 1101, 1102, 1103, . . . 11079 are carried within a 2.400 GHz to 2.482 GHz band 111. Each of the 79 channels are approximately 1 Mhz wide and are centered at frequencies 1 Mhz apart.
The first channel 1101 is centered at 2.402 Ghz, the second channel 1102 is centered at 2.403 Ghz, the third channel 1103 is centered at 2.404 Ghz, etc., and the seventy ninth channel 11079 is centered at 2.480 Ghz. The heterodyne demodulator 106 accurately receives a single channel while providing good suppression of the other channels present within the band 111. For example, if channel 1102 is the channel to be received, the baseband signal b(t) within channel 1102 will be presented while the baseband signals carried by channels 1101, and 1103 through 11079 will be suppressed.
An FSK modulation/demodulation approach is commonly used to transmit digital data over a wireless system. An example of an FSK modulation approach is shown in FIG. 1. A transmitting modulator 105 within a transmitting device 165 modulates a baseband signal at a carrier frequency fcarrier into an antennae 102. That is (referring to the frequency domain representation 150 of the signal launched into the antennae 102) if the baseband signal corresponds to a first logic value (e.g., xe2x80x9c1xe2x80x9d), the signal 150 has a frequency of fcarrier+fo. If the data to be transmitted corresponds to a second logic value (e.g., xe2x80x9c0xe2x80x9d), the signal has a frequency of fcarrierxe2x88x92fo.
Thus, the signal launched into the antennae 102 alternates between frequencies of fcarrier+fo and fcarrierxe2x88x92fo depending on the value of the data being transmitted. Note that in actual practice the transmitted signal 150 may have a profile 151 that is distributed over a range of frequencies in order to prevent large, instantaneous changes in frequency. The carrier frequency fcarrier corresponds to the particular wireless channel that the digital information is being transmitted within. For example, within the BLUETOOTH wireless system, fcarrier corresponds to 2.402 Ghz for the first channel 1101. The difference between the carrier frequency and the frequency used to represent a logical value is referred to as the deviation frequency fo.
Referring now to the heterodyne demodulator 106, note that the signal received by antennae 103, may contain not only every channel within the frequency band of interest 111, but also extraneous signals (e.g., AM and FM radio stations, TV stations, etc.) outside the frequency band 111. The extraneous signals are filtered by filter 113 such that only the frequency band of interest 111 is passed. The filter 113 output signal is then amplified by an amplifier 114.
The amplified signal is directed to a first mixer 116 and a second mixer 117. A pair of downconversion signals d1(t), d2(t) that are 90xc2x0 out of phase with respect to each other are generated. A first downconversion signal d1(t) is directed to the first mixer 116 and a second downconversion signal d2(t) is directed to the second mixer 117. Each mixer multiples its pair of input signals to produce a mixer output signal. Note that the transmitting modulator 105 may also have dual out of phase signals that are not shown in FIG. 1 for simplicity. Transmitting a pair of signals that are 90xc2x0 out of phase with respect to one another conserves airborne frequency space by a technique referred to in the art as single sideband transmission.
The frequency fdown of both downconversion signals d1(t), d2(t) is designed to be fcarrierxe2x88x92fIF. The difference between the downconversion frequency fdown and the carrier frequency fcarrier is referred to as the intermediate frequency fIF. Because it is easier to design filters 118a,b and 127a,b that operate around the intermediate frequency, designing the downconversion that occurs at mixers 116, 117 to have an output term at the intermediate frequency fIF enhances channel isolation.
The mixer 117 output signal may be approximately expressed as
kbFSK(t)cos(2xcfx80fcarriert)cos(2xcfx80fdownt).xe2x80x83xe2x80x83Eqn. 1
Note that Equation 1 is equal to
kbFSK(t)[cos(2xcfx80(fcarrierxe2x88x92fdown)t)+cos(2xcfx80(fcarrier+fdown)t)]xe2x80x83xe2x80x83Eqn. 2
which is also equal to
xe2x80x83kbFSK(t)cos(2xcfx80fIFt)+kbFSK(t)cos(2xcfx80(fcarrier+fdown)t)xe2x80x83xe2x80x83Eqn. 3
using known mathematical relationships. The bFSK(t) term represents a frequency shift keyed form of the baseband signal (e.g., a signal that alternates in frequency between +fo for a logical xe2x80x9c1xe2x80x9d and xe2x88x92fo for a logical xe2x80x9c0xe2x80x9d). The constant k is related to the signal strength of the received signal and the amplification of amplifier 114. For approximately equal transmission powers, signals received from a nearby transmitting device are apt to have a large k value while signals received from a distant transmitting device are apt to have a small k value.
Equation 3 may be viewed as having two terms: a lower frequency term expressed by kbFSK(t)cos(2xcfx80fIFt) and a higher frequency term expressed by kbFSK(t)cos(2xcfx80(fcarrier+fdown)t) Filter 118b filters away the high frequency term leaving the lower frequency term kbFSK(t)cos(2xcfx80fIFt) to be presented at input 119 of amplification stage 125. Note that, in an analogous fashion, a signal kbFSK(t)sin(2xcfx80fIFt) is presented at the input 126 of amplification stage 170.
Amplification stage 125 has sufficient amplification to clip the mixer 117 output signal. Filter 127b filters away higher frequency harmonics from the clipping performed by amplification stage 125. Thus, amplification stage 125 and filter 127b act to produce a sinusoidal-like waveform having approximately uniform amplitude for any received signal regardless of the distance (e.g., k factor) between the transmitting device and the receiving device.
After filter 127, a signal s(t) corresponding to AbFSK(t)cos(2xcfx80fIFt) is presented to the frequency to voltage converter 128 input 129 (where A reflects the uniform amplitude discussed above). The spectral content S(f) of the signal s(t) at the frequency to voltage converter 128 input 129 is shown at FIG. 1. The signal s(t) alternates between a frequency of fIF+fo (for a logical value of xe2x80x9c1xe2x80x9d) and a frequency of fIFxe2x88x92fo (for a logical value of xe2x80x9c0xe2x80x9d). The spectral content S(f) of the signal s(t) at the frequency to voltage converter 128 input 129 is mapped against the transfer function 160 of the frequency to voltage converter 128 in order to reproduce the baseband signal b(t) at the demodulator output.
Referring back to the pair of downconversion signals d1(t), d2(t) that are directed to mixers 116, 117, recall that the downconversion signals d1(t) and d2(t) should have a downconversion frequency fdown equal to fcarrierxe2x88x92fIF for each of the channels 1101 through 11079. For example, for an intermediate frequency fIF of 3 Mhz, the frequency synthesizer 140 is responsible for generating a frequency of 2.399 Ghz in order to receive the first channel 1101 (i.e, fcarrierxe2x88x92fIF=2.402xe2x88x920.003 Ghz=2.399 Ghz); a frequency of 2.400 Ghz in order to receive the second channel 1102; a frequency of 2.401 Ghz in order to receive the third channel 1103; . . . etc., and a frequency of 2.477 Ghz in order to receive the 79th channel 11079. A channel select input 141 presents an indication of the desired channel to the frequency synthesizer 140.
Both the transmitting device 165 and the receiving device 166 typically have a frequency synthesizer. A frequency synthesizer 140 is shown in the receiving device 166 (but not the transmitting device 165 for simplicity). Frequency synthesizers typically create their output signals by multiplying a reference frequency (such as the frequency of a local oscillator). As seen in FIG. 1, frequency synthesizer 140 multiplies the frequency of local oscillator 142 to produce downconversion signals d1(t) and d2(t). For example, for a local oscillator 142 reference frequency of 13.000 MHz, frequency synthesizer 140 should have a multiplication factor of 184.53846 to produce downconversion signals d1(t), d2(t) used to receive the first channel 1101 (i.e., 84.53846xc3x9713.000 MHz=2.399 GHz).
A problem with wireless technology involves deviation from the xe2x80x9cdesigned forxe2x80x9d carrier fcarrier and/or downconversion fdown frequencies (e.g., from non zero tolerances associated with the local oscillator 140 reference frequency). As either (or both) of the carrier and/or downconversion frequencies deviate from their xe2x80x9cdesigned forxe2x80x9d values, offsets may be observed in the baseband signal b(t) at the demodulator 106 output.
FIG. 2a shows a baseband signal 250 if the carrier and downconverting frequencies are ideal. As discussed above, the spectral content 253 of the signals produced by filters 127a,b will be centered at the intermediate frequency fIF. Since the origin 250 of the frequency to voltage converter transfer curve 260 is centered at the intermediate frequency fIF, the output signal 250 has no offset (i.e., has an offset positioned at 0/0 volts)
Errors in the carrier and/or downconversion frequency, however, will cause the spectral content of the signals produced by filters 127a,b to be centered at an offset 254 from the intermediate frequency fIF. That is, because fIF in equation 3 corresponds to fcarrierxe2x88x92fdown, if either fcarrier or fdown (or both) are in error the value of fIF in equation 3 does not correspond to the designed for fIF value (e.g., 3 Mhz) that is centered at the origin of the transfer curve 260. As such, the baseband signal 255 will have an offset 256 with respect to 0.0 volts.
A method that comprises reducing an offset in a baseband signal by changing a downconversion frequency in response to the offset.